Wednesday, August 9, 2017

by
Dr Hanno Slawski, Group R&D Director, Aller Aqua
Rainbow trout are rovers, their braveness to explore new habitats and their
adaptability to various environments has made trout a perfect species for
aquaculture

The trout as a pioneering species in the sector has also paved the way for
other fish species in terms of nutritional requirements, feed manufacturing,
farming systems and certifications. The knowledge obtained through research on
trout has led to a comprehensive understanding of nutritional and physical feed
quality as well as raw material quality and its impact on nutrient
digestibility, feed palatability and faeces quality. As far as applicable, the
knowledge obtained in research on trout has also been introduced into feeds for
other species.

Raw material nutrient digestibility
In recent years, feed for rainbow trout has undergone major changes in raw
material selection and composition. Institutional and industrial research has
led to an understanding of the nutritional requirements of trout that is only
matched in a few other fish species. As one example, it was possible to reduce
the dependency on marine raw materials in feed formulations, which used to be
the reference for nutritional quality.

However, raw materials provide nutrients to a feed. But not all nutrients from
a raw material are accessible for fish. This also counts for fishmeal and fish
oil. Some raw materials contain less accessible nutrients. Accessible nutrients
can also be described as digestible nutrients. Digestible nutrients are the
nutrients that are absorbed by fish from a feed between feed intake and faeces
excretion. Thus, digestible nutrients are ingested nutrients minus nutrients
excreted via faeces.

Consequently, the nutrients having value for fish are the ones it can digest.
Thus, a stable content of digestible protein and energy in the feed is more
relevant than the total content of protein and energy in the feed or the
selection of raw materials. Providing the optimum combination of digestible
protein and digestible energy is paramount for growth performance of fish.
Formulating feeds according to fixed levels of digestible protein and energy
outbalances varying numbers of total protein and total energy in a raw
material. Furthermore, formulating feeds according to nutrient digestibility is
the closest one can do to stabilise feed performance on a fish farm.

In rainbow trout, raw material nutrient digestibility is commonly determined in
feeding trials. Testing in vivo requires provision of various batches of test
feed. This also allows testing a raw material in production; its impact on feed
palatability and eventually faces quality. The test feeds are produced under
pilot scale and contain an inert marker. The marker is fully indigestible. It
passes the digestive tract of fish without affecting feed digestibility. From
the amount of marker found in the faeces one can calculate the amount of feed
it represents. Collection of faeces is therefore required. During the feeding
trials, faeces required for analysis are separated from the water column in
faeces collectors.

From there, the faeces are extracted and analysed for nutrient content and
concentration of the inert marker. The nutrient content of the faeces is then
compared with the nutrient content in the feed and a raw material under
investigation. Eventually the nutrients ingested by the fish and the nutrients
excreted via faeces can be compared.

This comparison gives so called Apparent Digestibility Coefficients (ADCs).
They are the targets of the digestibility trials. The ADCs allow comparisons
between nutrient digestibility of different raw materials.

In principle, ADCs for dry matter, protein, amino acids, fat, fatty acids, ash
and certain minerals are determined for each raw material from each supplier
and supplying factory. Once a catalogue for ADCs of all raw materials is
established, the feed formulation can be done based on nutrient digestibility.

Feed intake
Rainbow trout are voracious eaters. Under optimum environmental conditions,
trout have an enormous capacity for feed intake and feed digestion. Thus, the
faster and more the fish can eat, the quicker it will grow.

Raw material selection and physical feed quality can impact feed intake and
digestion rates. The physical quality of the feed is hereby closely linked to
the raw materials used in formulation. It has been observed for example, that
trout may voraciously eat a feed portion given in the morning. A second feeding
later that day may show a lack of appetite. Then, lack of palatability is
sometimes claimed to be the reason of reduced feed intake.

However, lack of palatability would have been apparent in the morning feeding
already. The sudden lack of appetite will more likely be linked to slow
digestion rate of the feed in the fish. Slow digestion rate is caused by
disadvantageous physical feed quality and reduced nutrient digestibility.

As an example, the surface structure, pore volume and pore distribution of a
feed pellet impacts its water absorption speed in the stomach of a fish. The
faster the pellet falls apart in the stomach the earlier feed particles enter
the gut for absorption of nutrients. Raw materials influence the pellet
porosity and consequently water stability, hereby impacting feed intake.Read the full article, HERE.

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